This application is a 371 application of International Application No. PCT/JP98/00439 filed Feb. 3, 1998.
The present invention relates to a carbonaceous electrode material for a secondary battery. More particularly, the present invention relates to a carbonaceous material having a large capacity for doping with a battery (or cell) active substance and suitable as an electrode material for a non-aqueous solvent-type secondary battery having a high energy density, and a process for production thereof.
Non-aqueous solvent-type lithium secondary batteries having a negative electrode comprising a carbonaceous material have been proposed as high energy density secondary batteries (Japanese Laid-Open Patent Application (JP-A) 57-208079, JP-A 62-90863, JP-A 62-122066, etc.). Such a secondary battery utilizes a phenomenon that lithium as a (cell) active substance easily dopes an carbonaceous material or is dedoped (i.e., released) from the carbonaceous material electrochemically. When the battery is charged, lithium in a positive electrode comprising a chalcogenide, such as LiCoO2, is introduced between layers of negative electrode carbon (i.e., dopes the carbon) electrochemically. The carbon thus doped with lithium functions as a lithium electrode, from which the lithium is released (i.e., de-doped) during discharge to return to the positive electrode. Thus, a secondary battery capable of repetitive charge-discharge is formed.
In case where an electrode is composed of graphite or a carbonaceous material having a developed graphite structure, a graphite intercalation compound is formed to enlarge the spacing between the graphite layers when the carbonaceous material is doped with lithium. When the lithium introduced between the layers is dedoped, the graphite layer spacing is restored to the original state. Accordingly, in a carbonaceous material with a developed graphite structure, the repetition of enlargement/restoration of the graphite layer spacing is caused corresponding to the repetition of charge/discharge of a secondary battery, whereby the graphite crystal structure is liable to be broken. Accordingly, a secondary battery constituted by using a carbonaceous material with a developed graphite structure has been said to have an inferior charge/discharge repetition performance. It is further said that a battery using such a carbonaceous material having a developed graphite structure is liable to cause decomposition of the electrolytic solution at the time of operation of the battery.
On the other hand, it has been also proposed to use carbonaceous materials obtained by calcining phenolic resins as negative electrode materials for secondary batteries (e.g., JP-A 58-209864, JP-A 62-122066, JP-A 63-276873). However, in case where a negative electrode is constituted by using a carbonaceous material obtained by calcining a phenolic resin at a high temperature of, e.g., 1900xc2x0 C. or higher, the resultant negative electrode is liable to have only small capacities of doping and dedoping of an active substance, such as lithium. Further, in case where a negative electrode is constituted by using a carbonaceous material obtained by heat-treating a phenolic resin at a relatively low temperature of, e.g., ca. 480-700xc2x0 C., the resultant negative electrode advantageously has a large capacity of doping with lithium as the active substance but is accompanied with a problem that the lithium doping the negative electrode cannot be completely dedoped to leave a substantial amount of the lithium in the negative electrode, so that the lithium as the active substance is wasted uselessly.
In view of the above-mentioned problems, an object of the present invention is to provide a carbonaceous electrode material for a secondary battery capable of providing a non-aqueous solvent-type secondary battery having large charge/discharge capacities and having little irreversible capacity determined as a difference between a doping capacity and a dedoping capacity, thus being capable of effectively utilizing an active substance.
Another object of the present invention is to provide a process for producing such a carbonaceous electrode material and a secondary battery using such a carbonaceous electrode material.
In the course of our study for obtaining a high-performance carbonaceous electrode material more suitably used in a non-aqueous solvent-type secondary battery, it has been found possible to obtain a carbonaceous material capable of providing a non-aqueous solvent-type secondary battery having large charge/discharge capacities and little irreversible capacity (i.e., a large active substance utilization rate) by using a phenolic resin having a controlled structure as a starting material and subjecting the phenolic resin to an appropriately controlled carbonization process.
More specifically, according to the present invention, there is provided a carbonaceous electrode material for a secondary battery, comprising: a carbonization product of an aromatic condensation polymer formed by condensation of an aromatic compound having a phenolic hydroxy group and an aldehyde; and having an atomic ratio H/C between hydrogen atoms and carbon atoms of below 0.1, a carbon dioxide adsorption capacity of at least 10 ml/g, and an X-ray scattering intensity ratio IW/ID of at least 0.25, wherein IW and ID represent scattering intensities as measured in a wet state and a dry state, respectively, at a parameter s=2xc2x7sin xcex8/xcex of 0.5 nmxe2x88x921, wherein xcex8 denotes a scattering angle and xcex denotes a wavelength of X-rays in X-ray small-angle scattering measurement.
The above-mentioned carbonaceous material according to the present invention has a large capacity for doping with an active substance of a secondary battery, such as lithium and has only a small value of so-called irreversible capacity, i.e., an amount of active substance caused to remain in the carbonaceous material without dedoping. By using such a carbonaceous material as an electrode material for constituting a non-aqueous solvent-type secondary battery, the resultant secondary battery is allowed to have large charge/discharge capacities and a high energy density.
The method for measuring the X-ray scattering intensity ratio IW/ID will be described later.
It is further preferred that the carbonaceous material according to the present invention has an average layer plane spacing between (002) planes according to X-ray diffraction (hereinafter sometimes denoted by d002) of at least 0.360 nm and at most 0.400 nm.
The carbonaceous material according to the present invention may preferably comprise a carbonization product of a resol-type phenolic resin, which is a condensation product between a phenol and an aldehyde. More specifically, the resol-type phenolic resin includes a condensation product initially obtained by reaction between an aromatic compound having a phenolic hydroxyl group and an aldehyde in the presence of a basic catalyst, and a resinous substance obtained by thermal curing of such an initial condensation product.
The carbonaceous material according to the present invention may be produced in the following manner.
More specifically, the carbonaceous material according to the present invention may be produced by carbonizing an aromatic condensation polymer, which is a condensation product between an aromatic compound having a phenolic hydroxyl group and an aldehyde, at a temperature of 1050-1400xc2x0 C. under a pressure exceeding 10 kPa (=0.1 atm) while flowing an inert gas.
The carbonaceous material according to the present invention may also be produced by carbonizing an aromatic condensation polymer, which is a condensation product having a phenolic hydroxyl group and an aldehyde, at a temperature of 1050-1400xc2x0 C. under a pressure of at most 10 kPa.
Herein, the inert gas may include an inert gas, such as nitrogen gas, argon gas or helium gas, and a gaseous mixture comprising such an inert gas, and a halogen gas, such as chlorine gas, in an amount of at most 40 mol % of the gaseous mixture.
The non-aqueous solvent-type secondary battery according to the present invention comprises a positive electrode, a negative electrode, and a separator and a non-aqueous electrolytic solution disposed between the positive and negative electrodes; at least one of the positive and negative electrodes comprising a carbonaceous material according to the present invention as described above.